m5board.comoholic (>1000 posts)
Join Date: Nov 2010
Location: Fairfax, VA USA
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M5 London may have some insight, however, I do not think anyone other than the DME programmers really know what triggers the Coolant Temp Plausibility code?
I had just replaced my thermostat in the Winter and was actually watching my temps like a hawk as I was curious to see how the car temps behaved in the very cold weather. My coolant temp sensor was replaced a few months before I changed the stat by the prior owner, so it was relatively new. I actually had the OBC hidden menu temps up while driving for a 2-3 week period all the time. Car was fully warmed up and I was driving on a 55 MPH stretch with slight downhill grade for about 3-4 miles, then came to a stop. Took off from the stop and bang, CEL/MIL light comes on.
Get home and check the codes, Coolant Temp Plausibility and Cat Inefficiency codes. I call 100% BS on this as I was watching the temps for weeks and really closely just before the code was triggered. Turns out that I have slight post Cat exhaust leak causing Cat Inefficiency codes. I believe the way the DME is programmed, it may trigger Coolant Temp Plausibility for certain situations where the engine may not be warm enough, like Cat Inefficiency??
So before you get too concerned, watch OBC hidden menu #7 closely and see if you have any other codes either pending or triggering at the same time.
Just be careful not to believe everything you see as far as CEL/MIL codes, many times they are not 100% accurate, but may give you an idea as to what is up. My Cat Inefficiency codes are due to a slight post Cat exhaust leak, not bad Cats. Search the Internet and you will see countless Cats being replaced on these cars and many other non BMW's and the codes do not go away. Many times 02 sensors, minor exhaust leaks and exhaust mods cause these codes.